Gamma rays have been used to measure the density and level of fluids in a vessel by using a gamma-ray source positioned opposite a gamma-ray detector. These through-transmission gamma-ray density and level measurements are useful where the materials measured are hazardous, extremely hot, or where direct contact measurements are otherwise not possible. Additionally, the source and detector are mounted outside the vessel, and no modification to the vessel is required. Gamma rays emitted by a source may be absorbed or attenuated by the vessel and the material in the vessel. The strength of the gamma radiation reaching a detector opposite the source may be used to indicate the density or level of a fluid in a vessel based upon the intensity of the source.
When measuring fluid level, for example, multiple gamma-ray emitters and/or detectors may be positioned at opposite sides of a vessel, where the presence or absence of a signal (or a nominal low signal) may indicate the presence or absence of a fluid in place between the source and detector. The size of a vessel in a signal/no signal level detector may be much larger than that for a gamma-ray densitometer, as described below, as gamma rays are not as readily absorbed or attenuated by vapors in the vessel.
With respect to fluid density, for example, fluid passing between the gamma-ray source and detector may absorb or attenuate gamma rays emitted by the source. A high radiation count indicates a low fluid density while a low count indicates high fluid density.
However, through-transmission density measurement using gamma rays is viable only for limited vessel sizes and/or fluid densities. For example, for a similar sized source, at higher fluid densities, the fluid may absorb more gamma rays, thus resulting in fewer gamma rays reaching the detector. Similarly, as vessel size is increased, gamma rays must pass through a greater quantity of material (vessel and fluid) absorbing the gamma rays, resulting in fewer gamma rays reaching the detector. Therefore, gamma-ray density measurements in this manner are currently only viable for vessels up to about 1 meter in diameter.
Another disadvantage in the present use of gamma rays for through-transmission density measurements is that the solid angle subtended by a fixed size detector, and thus the counting rate, scales inversely with the size of the vessel squared. The counting rate n may be approximated by the equation:n˜Ωe−d/λ˜(e−d/λ)/d2  (1)where n is the counting rate, d is the vessel diameter, and λ is the absorption length, which depends on density. For a fixed sized detector, an increase in the vessel diameter d results in a lower count rate and a greater rate of error. Accordingly, for large vessels in noisy environments, it may become impossible to distinguish the gamma ray signal from the spurious background signal and, thus, useful information cannot be extracted.
To overcome the thickness, size, and density limitations, the intensity of the gamma-ray source may be increased, thus resulting in a measurable quantity of gamma rays reaching the detector. However, cost, safety, multi-unit effectiveness, and security may each limit the source intensity that may be used. For example, the use of a radioactive source creates personnel safety and environmental concerns and requires lead or tungsten shielding to protect personnel, special handling precautions and equipment, as well as disposal and remediation procedures. Furthermore, because gamma rays are produced from a point source and not a directional source, as the size of the source increases, the amount of shielding required to contain the radiation in directions other than through the vessel must be increased, thus, adding further to the cost.
With respect to multi-unit effectiveness, a chemical plant may desire to use gamma-ray level and density gauges on multiple vessels. However, as the number of gauges is increased or the intensity of gamma-ray sources is increased to overcome size limitations, cross-talk between gamma-ray sources and detectors on adjacent vessels may occur, resulting in decreased effectiveness and potentially erroneous readings.
With respect to the problem of measuring a density profile, i.e., the density as a function of elevation in a vessel, similar problems arise. For example, when attempting to use multiple units on a single vessel in order to estimate density profiles of the fluid contained in the vessel, size limitations and cross-talk between gamma-ray sources make existing technology incapable of producing accurate and reliable density profile measurements.
Regarding security, due to growing worldwide concerns about the proliferation and possible smuggling or other transport of radioactive materials, state, local, and national governments regulate facility security requirements based upon the total amount of radioactive material that may be present at a single site. For example, the State of Texas requires additional security measures (e.g., background checks, accessibility, etc.) at facilities where the total Curie count exceeds 27 Curie, where the total Curie count is based upon a sum of all radioactive sources at the facility. Thus, use of larger sources to overcome vessel size limitations may result in an increased need for security at an additional cost.
Accordingly, there exists a need for gamma-ray density gauges that may be used on larger vessels. Additionally, there exists a need for non-contact density gauges that require lower intensity radiation sources. Additionally, there exists a need for non-contact density gauges that can measure the density profile of the fluid in addition to the density at a single location in a vessel.